All templates fail to build: unpinned devcontainers/base:ubuntu rolled to 26.04, breaking docker-in-docker

All four templates fail to build. The `docker-in-docker` feature aborts during install:

```
(!) The 'moby' option is not supported on ubuntu 'resolute' because 'moby-cli' and related
system packages are not available in that distribution.
(!) To continue, either set the feature option '"moby": false' or use a different base image.
ERROR: process "/bin/sh -c ... ./install.sh" did not complete successfully: exit code: 1
```

## Cause

Every template pins the base image to the floating `:ubuntu` tag:

- `src/full-stack/.devcontainer/devcontainer.json:3`
- `src/node-agentic/.devcontainer/devcontainer.json:3`
- `src/python-agentic/.devcontainer/devcontainer.json:3`
- `src/rust-agentic/.devcontainer/devcontainer.json:3`

That tag has rolled forward to Ubuntu "resolute" (26.04). Canonical does not publish `moby-cli` or the related Moby packages for that release, and `ghcr.io/devcontainers/features/docker-in-docker:2` defaults to `moby: true` — so it refuses to install. All four templates include docker-in-docker, so all four are affected. Nothing in the templates changed; the tag moved underneath them.

## Suggested fix

Pin to the current LTS:

```diff
- "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+ "image": "mcr.microsoft.com/devcontainers/base:ubuntu-24.04",
```

Setting `"moby": false` also unblocks the build, but it swaps Moby for upstream Docker CE and leaves the templates on a pre-release distro where the other features are free to break on the next tag move. Pinning addresses the cause and makes builds reproducible; 26.04 can then be a deliberate bump once moby packages land.

Verified locally in a downstream project derived from `python-agentic` — pinning to `ubuntu-24.04` builds clean through all features.

Update the image entries at line 3 of src/full-stack/.devcontainer/devcontainer.json, src/node-agentic/.devcontainer/devcontainer.json, src/python-agentic/.devcontainer/devcontainer.json, and src/rust-agentic/.devcontainer/devcontainer.json. Start by checking each template's current base image, pin it to ubuntu-24.04, then build all four templates and confirm docker-in-docker and the other features install successfully.

Written by the indexing model from the issue text.